Dr. Remy "Thirteen" Hadley was concerned she would develop Huntington's Disease as her mother died from the disorder. Huntington's is caused by a single defective gene and a parent has a 50/50 chance of passing it on to a child. Genetic tests can determine whether a child of a person with the disease will develop it. However, Thirteen consistently refused to take the test. She did take the test at the end of season 4 and it was positive. In season 5 while taking part in a clinical trial, Dr. Foreman tested her and found that she was suffering from the early stages of the disease.